A semiconductor laser and other semiconductor light emitting device are used, for example, as a CD (compact disc) and DVD (digital versatile disc), furthermore, a light source of an optical pickup device of next-generation optical disc devices and a light source of other apparatuses in a variety of fields.
As the above semiconductor light emitting device, for example, a semiconductor laser made by an AlGaInP-based material is disclosed in the non-patent article 1.
FIG. 1A is a sectional view of the semiconductor laser explained above.
For example, an n-type cladding layer 111 formed by an AlGaInP layer, an active layer 112, a p-type cladding layer 117 formed by AlGaInP layers (113 and 115) and a p-type cap layer 118 formed by a GaAs layer are formed by being stacked on an n-type substrate 110 via a not shown n-type buffer layer.
An etching stop layer 114 of a GaInP layer is formed on a boundary face of the AlGaInP layer 113 and the AlGaInP layer 115, and a portion from a surface of the p-type cap layer 118 to the AlGaInP layer 115 is processed to be a ridge (protrusion) shape RD so as to form a stripe as a current narrowing structure.
Current block layers 119 are formed on both sides of the ridge shape RD and, furthermore, a p-electrode 120 is formed to be connected to the p-type cap layer 118 and an n-electrode 121 is formed to be connected to the n-type substrate 110.
FIG. 1B is a view of a bandgap profile of a section along x1-x2 in FIG. 1A.
It shows a bandgap of each of the n-type cladding layer 111, active layer 112, AlGaInP layer 113, etching stop layer 114 and AlGaInP layer 115.
For example, a composition ratio of aluminum in the n-type cladding layer 111 is 0.65, while that in both of the AlGaInP layers (113 AND 115) is 0.70 and p-type cladding layers are configured to have a higher bandgap than that of the n-type cladding layer 111.
In the above semiconductor laser, to adjust an aspect ratio of the laser beam and bring the beam shape close to a circular shape is one of significant tasks.
The beam shape largely depends on a refractive index of each layer composing the semiconductor laser.
On the other hand, in the conventional semiconductor laser explained above, a variety of attempts have been made to improve the internal quantum efficiency and two leakage currents are required to be minimum.
A first leakage current is a lateral direction leakage current ILx, which leaks excessively in the X-direction parallel to a hetero junction in the sectional view in FIG. 1. A second leakage current is a longitudinal direction leakage current ILy called an overflow, wherein electrons leak in the Y-direction from the active layer to the p-cladding layer.
There is a method of controlling the ILx by making a thickness of the AlGaInP layer 113 in FIG. 1 thin, however, it is actually difficult to make the AlGaInP layer 113 thin by controlling to 300 nm or thinner.
For example, a difference of an effective refractive index Neff1 at the center of the ridge stripe and an effective refraction index Neff2 outside of the ridge stripe becomes large, light confinement in the X-direction intensifies, a photon distribution at the center in the X-direction is maximized, and electron-hole consumption increases to be short in supply. This is called hole-burning of carriers and photons are unable to be supplied with electron holes to maintain the mode at this time, so that they tend to shift to a mode of receiving the supply. This phenomenon leads to a change of electron-light conversion efficiency thereof, and linearity of the light output—current (L-I) characteristic is deteriorated, which is observed as a phenomenon called kink.
Also, in the conventional semiconductor laser explained above, electrons leak as a longitudinal direction leakage current ILy from the active layer to the p-type cladding layer due to the thermal electron energy when at a high temperature and deterioration of the L-I characteristics is caused.
As a nature of countermeasures thereof, a method of heightening a height of an energetic barrier sensed by electrons belonging to a Γ-band and a method of improving concentration of a p-type impurity of the cladding layer have been general. At this time, it is known that a drift current of an electron group belonging to an X-band increases when the AlGaInP layer 113 is made thinner as a significant task (refer to the non-patent article 1).
This can be confirmed also by an experiment and the AlGaInP layer 113 cannot be made very thin, so that a method of controlling the leakage current ILx in the X-direction explained above cannot be used.
Non-Patent Article 1: Numerical Simulation of Semiconductor Optoelectronic Devices, proceedings, MD4, L39-40
Non-Patent Article 2: IEEE JQE, vol. 38, No. 3, March 2002, L285.